EE 5181: Introduction to Nanotechnology
Lab Experiment 4: Elastomeric stamping and phase shift lithography
Introduction:
In this lab, we will be exploring a selection of the various uses for the
elastomeric PDMS stamps that we casted in lab 3. To begin, you will peel the
PDMS stamp from your mold. You will then carry out microcontact printing (µCP)
of a self-assembled monolayer (SAM), solvent assisted micromolding (SAMIM),
channel outgassing technique (COT), and near-field phase-shift lithography
(NFPSL) using your stamps as well as those provided for you. By the end of the
session, you will have completely fabricated a PDMS stamp from a given mold
and will have familiarity with a sample of soft lithography techniques. Additionally,
you will be further exposed to the Nanofabrication Center (NFC) and
photolithography tools including spin coaters, mask aligners, and wet benches.
Self Assembled Monolayer (SAM) Microcontact Printing (µCP)
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“Ink” the PDMS stamp (yours or one of the several provided for you) with a
SAM solution by immersion. In this case, we will be using 10 millimolar 1dedecanethiol (hydrophobic) and 10 millimolar 16-mercapthexadecanoic
acid (hydrophilic) each separately in ethanol.
Rinse the stamp with ethanol and dry with a nitrogen stream.
Transfer the stamp pattern by bringing it into contact with a cleaned gold
wafer surface. Allow the stamp to conformally coat the surface.
Being careful not to smudge the pattern, remove the stamp after 20
seconds.
To verify that the pattern was indeed transferred, breathe on the gold
wafer. The water vapor in your breath will condense on the sample
preferentially in the more hydrophilic areas. An ice block can be used to
slow evaporation.
Alternatively, the PDMS stamp can be mounted to the bottom of the drill
press stand and the stamp lowered into contact by pulling the side lever.
With this method of CP care must be taken to not add excessive force or
the wafer may crack.
Water collecting on hydrophilic areas defined by the µCP process (15µm gears)
In your report, discuss the degree and importance of drying the stamp
before successful pattern transfer. Describe the potential benefits and drawbacks
of using a press to bring the stamp in contact with the substrate. Include a
sentence about the necessity of having a clean gold substrate. Finally, describe
how you could use this process to fabricate gold lines/features on silicon.
Solvent-Assisted Micromolding (SAMIM)
Your elastomeric stamp can also be used for micromolding. Recall that in
our previous lab, we used a hard master to emboss a thermally sensitive resist
by applying both heat and pressure to the two. The SAMIM process replaces the
heat and pressure with a solvent. Here, we will be using ethanol-soaked stamps
to pattern 1805 photoresist coated substrates.
Nanoimprinter:
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SAMIM Process:
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Prepare a silicon substrate with 500nm of 1805 photoresist (provided).
Soak your PDMS stamp in ethanol (overnight).
Dry the stamp with a nitrogen stream.
Transfer the stamp pattern by bringing it into contact with the photoresistcovered silicon substrate. Allow the stamp to conformally coat the surface.
Being careful not to smudge the pattern, remove the stamp after 10
seconds.
Observe the transferred pattern under the microscope.
Note: a reactive ion etcher (RIE) would be used, as with the
Nanoimprinter, to “descum” the bottoms of the new trenches.
1805 Photoresist defined by SAMIM (15µm gears)
In your report, describe the degree and importance of drying the stamp
before pattern transfer. Again, describe the benefits and drawbacks of using a
press and how this impacts resolution. How often could this process be
repeated? Finally, explain why a descum step is required based on what you
observe through the microscope.
Channel Outgassing Technique (COT)
Similar to the micromolding in capillary (MIMIC) process described in
class, you can also use your stamps to mold photosensitive polymer structures.
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Cut a PDMS stamp that has a channel running from one end, “A”, to
another, “B” (tortuous paths are allowed).
Allow the stamp to make conformal contact to a microscope slide by
bringing them into contact.
Dispense a small amount of NOA 73 optically sensitive adhesive at
channel end “A” on the glass slide, leaving channel end “B” exposed.
Place substrate, stamp, and NOA 73 into a vacuum desiccator.
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Pump down the desiccator using a rotary pump while observing the NOA
as it travels along the channels formed by the conformally contacted
microscope slide and PDMS stamp.
Vent the desiccator and remove the sample when the desired channel
length has been filled.
Expose the sample to UV light for 1 minute using a handheld source to
cure the NOA.
Peel back the stamp to reveal the COT produced NOA pattern beneath it.
Jennifer Monahan, Andrew A. Gewirth, and Ralph G. Nuzzo Anal. Chem. 2001
In your report, discuss the advantages and disadvantages of using COT
versus SAMIM. Include an explanation of why a descum step may not be
required.
Near Field Phase-Shift Lithography (NFPSL)
Your PDMS stamp can also be used to generate very small features in
photoresist using near field phase-shift lithography. The advantage of this
process is that it defines features at the edge of your stamp’s features, allowing
you to generate patterns that are potentially much small than your original
features.
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Prebake (dehydrate) a silicon wafer at 115°C for 2 minutes.
Treat the wafer in Hexamethyldisilazane (HMDS) vapor for 2 minutes.
Spincoat 1805 photoresist at 3000 RPM for 30 seconds.
Soft bake (solvent drive out) at 105°C for 1 minute.
Apply phase shift stamp feature side down on the substrate (conformal).
Affix phase shift stamp to blank glass mask plate.
Expose to UV light for 4 seconds in MA6 aligner.
Develop for 15 seconds in 351 (1:5) developer.
Rinse in DI water.
Dry under nitrogen stream.
Observe under microscope.
Calculated and observed diffraction patterns resulting from near field phase-shift lithography at
left, resulting photoresist structures at right.
Maria, Joana Malyarchuk, Viktor White, Jeff Rogers, John A. J. Vac. Sci. & Tech. B. 2006
In your report describe how near field phase shift lithography compares to
electron beam lithography, especially in speed and cost. Also include your
impressions on the photolithography bay in the NFC and any questions you may
want answered about it.